1. Technical Field
The present invention relates to an optical fiber coupling part, namely, an optical fiber with a lens for coupling a light emitting source such as a semiconductor laser used for optical communication and an optical fiber with high coupling efficiency, and a manufacturing method thereof.
2. Description of Related Art
A technique for coupling a semiconductor laser and an optical fiber is one of the most important techniques in optical communication. For example, conventional methods of coupling the semiconductor laser and the optical fiber include a method using a tip ball fiber whose tip part is spherical (see U.S. Pat. No. 3,910,677), or a method using a convex lens such as a spherical lens or an aspherical lens.
Since formed in small size, the method using the tip ball fiber is capable of coupling the semiconductor laser array and an optical fiber array. This tip ball optical fiber is integrally formed with a hemispherical lens part at the tip of a single mode optical fiber. Meanwhile, when making the tip ball part of the optical fiber, a problem is that conventionally the tip part of the fiber is polished all around, and therefore mass-productivity is deteriorated and it takes significant labor hours to produce. Another problem is that, since the tip of the optical fiber is spherical, coupling efficiency is deteriorated due to spherical aberrations. Specifically, a light beam emitted from a laser end face reaches the end face of the single mode optical fiber at different positions and at different angles, depending on the exit angle of the outgoing light. Therefore, some of the light beams deviate from the core, or even when it reaches the core, an incident angle to the core is equal to or larger than a critical angle, and therefore the light is not propagated through the single optical fiber so as to deteriorate coupling efficiency. For example, when a standard semiconductor laser is used, the coupling loss is approximately 6 dB.
On the other hand, the method using a convex lens involves problems that mutual alignment of the optical axes among a semiconductor laser 2, lenses 3, 4 of the semiconductor laser, the lens, and the optical fiber 1 is complicated, thereby increasing the manufacturing cost, while relatively high coupling efficiency is obtained, as shown in block diagrams of FIGS. 1 (a) and (b). FIG. 1(a) is a view illustrating optical coupling wherein outgoing light from the semiconductor laser 2 is adjusted to be a light 5 with an angle that is receivable by the optical fiber 1, by the convex aspheric lens 3, and light is condensed and introduced to a core 1a of the optical fiber 1. FIG. 1(b) is a view illustrating optical coupling wherein outgoing light from the semiconductor laser 2 is parallelized by the convex lens 3, and the parallel light beam 5 is condensed and introduced into the core 1a of the optical fiber 1, by the opposing counter lens 4. In FIG. 1(b), 2a denotesan active layer of the semiconductor laser 2, 6 denotes a supporting stand, 7 denotes a XYZθ stage, and 8 denotes a table.
The construction of FIG. 1(a) is a common construction currently adopted in optical communication using the DFB (distributed feed back) semiconductor laser. Especially in using the DFB semiconductor laser, an optical isolator (not shown) should be inserted between the convex aspheric lens 3 and the optical fiber 1 for preventing reflected light from the optical fiber from returning to the semiconductor laser, and therefore generally adopted from the viewpoint of saving space. How ever, if an ideal optical axis common to the semiconductor laser 2, the convex aspheric lens 3 and the optical fiber 1 is represented by a broken line C, and a direction parallel to the optical axis C is prescribed to be the Z direction, a direction perpendicular to the horizontal direction to be the X direction and a direction perpendicular to the vertical direction to be the Y direction, imperfect alignment in the end face of the optical fiber 1 is attributed to displacement in the X direction, a tilt angle θx in the X direction, displacement in the Y direction, a tilt angle θy in the Y direction and displacement in the Z direction. Especially in such an optical system, light of the semiconductor laser is condensed smaller on the end face of the core 1a (radius: about 6 μm) of the optical fiber 1 by the aspheric lens 3, and each optical axis should be therefore conformed with submicron precision for high coupling efficiency, so that it usually took a dozen minutes to adjust the cores, and the cost of manufacturing was significantly increased.
Although the optical isolator can be reliably inserted even in the construction of FIG. 1(b), the number of optical components for alignment of the optical axes is increased, and it takes a significant amount of time to adjust the cores under these circumstances. This was a factor increasing the cost of manufacture.
The lens 4 is commonly called a collimator lens. If the lens 4 and the optical fiber 1 are united with optical axes being mutually conformed (fusion splice: optical fiber with collimator lens), imperfect alignment at the end face position of the optical fiber 1 is attributed to only displacement in the X direction, the tilt angle θx in the X direction, displacement in the Y direction, and the tilt angle θy in the Y direction (displacement in the Z direction is absent), consequently alignment of the optical axes of the lens 4 and the optical fiber is unnecessary. Especially in such an optical system, the divergence radius of the parallel light beam 5 is several tens of micrometers and is broad, and coupling efficiency is therefore enhanced even with several micron precision of alignment of the optical axes, so that its productivity is obviously improved to several tens of times in comparison with the optical system of FIG. 1(a).
The lens 4 of FIG. 1(b), called a collimator lens, usually employs a columnar distributed index lens (Graded Index lens: hereinafter “GRIN lens”) because of common ease in attachment. In the GRIN lens shown in FIG. 2, when the refractive index n in the cross-section directions X and Y is represented by the following equation (1), the refractive index in the columnar center axis is highest, and the farther the point leaves from the center axis to the periphery, the lower the refractive index continuously becomes, in a quadratic curve (parabolic curve). Operation of the lens is carried out by this refractive index distribution.n=n0{1−g2r2/2}  (1)In this equation, g is a constant expressing a light-condensing performance (refractive index distribution constant) of the GRIN lens, n0 is the refractive index (refractive index of the center part) of the material of the GRIN lens, and r is a radial direction (r2=x2+y2). In FIG. 1, if the radius of the GRIN lens is a, and the refractive index at the radius a is na, g is represented as follows.g=NA/an0, wherein NA=(n02−na2)1/2  (2)In this equation, NA is square root of square-difference between the refractive indexes of the center and the periphery in the GRIN lens, which is called Numerical Aperture (hereinafter “NA”), and is an important parameter presenting lens performance. A high NA lens has good lens characteristics that are high light focusing abilities.
The length of the GRIN lens used as the collimator lens is set as follows, if the cycle length of ¼ as long as a zigzag cycle of a light propagated through the GRIN lens is L¼.L¼=π/(2g)  (3)Alternatively, the length may be prescribed to be an odd-number of times the length of ¼ of the zigzag cycle.
Further, the GRIN lens is conventionally made of a multi-component glass, and its softening point is about 500 to 600° C. Therefore, such a GRIN lens can not be fusion-spliced with the optical fiber, which is mainly composed of quartz glass. Thus, an optical adhesive is used, thereby posing problems in that it is difficult to align the opticalexes, and an optical characteristic is deteriorated by a change in the quality of the adhesive caused by temperature-rise, when the adhesive absorbs the light and high intensity light thereby enters. Consequently, unification of the convex lens 4 and the optical fiber 1 with optical axes mutually conformed (optical fiber with collimator) was impossible.
In order to solve such a problem of connection deterioration, a structure using GI (Graded-Index) optical fiber as a lens has been proposed, as disclosed in U.S. Pat. No. 4,701,011 and U.S. Pat. No. 5,384,874. The GI optical fiber is the optical fiber in which the refractive index of a core part changes in a radial direction. Since the GI optical fiber is made of the same quartz as the optical fiber, the GI optical fiber can be fusion spliced with the optical fiber. Therefore, it can be expected that the GI optical fiber will have high durability against light of high intensity. However, common GI optical fiber is made by the gas phase CVD (Chemical Vapor Deposition) method. In the gas phase method, operability is inefficient in actual unification as the collimator lens, in term of alignment of thermal expansibility like that makes the base material fragile due to increased coefficient of thermal expansion when amounts of the additives (GeO2, P2O5 or the like) are increased, or in term of controllability of the refractive index.
Patent document 1: U.S. Pat. No. 3,910,677
Patent document 2: U.S. Pat. No. 4,701,011
Patent document 3: U.S. Pat. No. 5,384,874